光合作用还没停，橡树为何提前“躺平”？
It is natural to think that when a tree is photosynthesizing—using sunlight, water, and carbon dioxide to make food—it is also growing. However, a study of 137 oak sites across the U.S., published in Science Advances, reveals that growth can stop by mid-summer, even though photosynthesis continues into autumn.
This matters for climate change. Forests absorb carbon dioxide and store carbon in wood for decades or centuries, slowing warming. Most models assume higher carbon dioxide boosts both photosynthesis and wood production, but the study shows extra photosynthesis does not always mean more wood.The carbon may go instead to short-lived leaves and fruits.
So where does the carbon go? Some helps the tree restart growth next spring. Some builds new leaves and roots. Some is released into soil. Less ends up in long-term wood storage than scientists assumed.
How do researchers know? Satellites tracked photosynthesis, while trunk sensors detected tiny daily changes: trees swell (膨胀) at night as roots absorb water, then shrink slightly by day as they lose moisture (水分). The long-term swelling trend is growth. In eastern U.S. oaks, growth ran from May to July, but photosynthesis lasted until October. About 36% of annual carbon uptake happened after growth stopped. In California, growth ended by August, with 26% of uptake occurring later.
Why the gap? When weather turns hot and dry, trees lose the internal water pressure that expands their cells, so growth stops. Photosynthesis, however, continues at a slightly reduced rate.